In order to exploit the available frequency spectrum more efficiently, current wireless communication standards require a tight arrangement of communication channels in the frequency domain. As a result, transmit energy from a nearby transmitter (which may be referred to as a transmit blocker signal) can interfere with the receive signal due to the proximity of the transmit antenna to one or more receive antennas. Current digital signal processing techniques exist that enable receivers to filter out a desired signal from a relatively strong noise floor (i.e., a relatively low signal-to-noise (SNR) ratio). However, a so-called blocker signal, interferer signal, or interference signal that is at a nearby frequency and has a signal level comparable to or even higher than the desired signal, poses a challenge to most receiver structures. In particular, a receiver module needs to process the transmit blocker signal and the desired signal with relatively high fidelity, so that the transmit blocker signal may be separated from the desired signal.
Transmitter nonlinearity generates unwanted spectrum outside the main channel. Some of the unwanted spectrum falls in the receive band and causes receiver de-sense in full duplex systems. A filter in the transmit path is used to attenuate the unwanted spectrum to an acceptable level at the receive band frequencies, but the filter also has insertion loss in the transmit band. For a given filter technology there is an unavoidable trade-off between selectivity and insertion loss.
Accordingly, it would be desirable to reduce the selectivity required in the filter by partially cancelling the unwanted transmit spectrum without the costs and disadvantages of the filters.